Solvent extraction with recycle of light nonaromatic fraction



SOLVENT EXTRACTION WITH- RECYCLE OF LIGHT NON-AROMATIC`FRACTION Filed Feb. 2e, 196e United States Patent O U.S. Cl. 208-318 7 Claims ABSTRACT OF THE DISCLOSURE Process for separating and recovering aromatic hydrocarbons such as benzene from a feed mixture via an improved solvent extraction technique. The preferred solvent comprises sulfolane and the extraction zone is refluxed with specific nonaromatic hydrocarbon streams.

BACKGROUND OF THE INVENTION The invention relates to a separation process. It also relates to a solvent extraction ,process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures utilizing a selective solvent.

The technique of aromatic hydrocarbon separation by solvent extraction is well known in the prior art. Generally, according to a well known procedure, a mixture of hydrocarbons is introduced into an extraction column at an intermediate point thereof and contacted with a solvent which is selective for aromatic hydrocarbons. The conditions in the extraction column are sufficient to produce a raffinate phase comprising substantially all of the non-aromatic hydrocarbons in the feedstock and an extract phase comprising the aromatic components of the feedstock dissolved in the selective solvent. Typically, the non-aromatic raffinate phase is recovered and utilized in gasoline blending. The extract phase is subsequently passed through various separation means for the recovery therefrom of aromatic hydrocarbons in high concentration and high purity. Lean solvent generally s uitable for reuse in the extraction zone is also recovered in the process for segregating the aromatic hydrocarbons.

One feature of the prior art process is the freeing of the extract phase of substantially all of the non-aromatic components dissolved by or carried by the solvent. This is accomplished by introducing a non-aromatic hydrocarbon reflux stream, typically comprising naphthenes and parains into the extraction zone at a point intermediate between the feed introduction point and the extract phase withdrawal point. Conventionally, this reflux phase is more volatile in the presence of the solvent than either of the aromatic or raffinate components of the feedstock and, in effect, acts as a displacing agent for the non-aromatic hydrocarbons out of the extract phase and into the raflinate stream.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a separation process.

It is another object of this invention to provide a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures.

It is still another object of this invention to provide an improved process for the recovery of aromatic hydrocarbons via a solvent extraction technique in a facile and economical manner. d

Therefore, the present invention provides a process for separating aromatic hydrocarbons from a feed mixture containing aromatic and non-aromatic hydrocarbons which comprises: (a) introducing said feed mixture into a solvent extraction zone in contact with lean solvent 3,487,013 Patented Dec. 30, 1969 ICC and hereinafter specified reflux stream under conditions sufficient to produce a rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing non-aromatic hydrocarbons; (b) removing from said zone the raffinate stream; (c) introducing said extract stream into a first distillation zone maintained under distillation conditions; (d) withdrawing from said first distillation zone a first overhead fraction containing solvent and non-aromatic hydrocarbons and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein; (e) passing said first overhead fraction into a second distillation zone maintained under conditions suicient to produce a distillate fraction containing non-aromatic hydrocarbons and a second bottoms fraction comprising solvent; and (f) returning said distillate fraction to said extraction zone as the specied reflux therein.

AS a broad general class, suitable feedstocks for the satisfactory practice of this invention include iiuid mixtures having a sufficiently high concentration of aromatic hydrocarbons to economically justify recovery ofv these aromatic hydrocarbons as a separate product stream. The ipresent invention is particularly applicable to hydrocarbon feed mixtures which contain at least 25% by weight aromatic hydrocarbons. A suitable carbon number range for the feedstock is from about 6 carbon atoms per molecule to about 2() carbon atoms per molecule and, preferably, from about 6 to 10 carbon atomsper molecule. One source of feedstock is the debutanizer reactor effluent from a conventional catalytic reforming process unit. Another source of satisfactory feedstock is the liquid byproduct from a pyrolysis gasoline unit which has been hydrotreated to saturate oleiins and diolens, thereby producing an aromatic hydrocarbon concentrate suitable for the solvent extraction technique described herein. Typically, the feedstock from a catalytic reforming process unit contains single ring aromatic hydrocarbons comprising a wide boiling mixture of benzene, toluene, and xylenes. These single ring aromatic hydrocarbons are mixed with the corresponding parafiins and naphthenes which have been produced from such a catalytic reforming unit.

The preferred solvent which may be utilized in the present invention is a solvent of the sulfolane-type. This solvent is well known to those skilled in the art and, typically, possesses a five membered ring containing one (1) atom of sulfur and four (4) atoms carbon with two (2) oxygen atoms bonded to the sulfur atom of the ring. Since this solvent is an article of commerce and is Well known to those versed in the solvent extraction art, greater detail thereof need not be presented at this time.

Other solvents which may be included and may satisfactorily be used in the practice of this invention are the sulfolenes, such as 2-sulfolene and 3-sulfolene. Still other typical solvents which have a high selectivity for separating aromatic hydrocarbons from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are Z-methyl-sulfolane, 24,dmethylsulfolane, methyl-Z-sulfonylether, naryl3 sulfonylamine, 2-sulfonylacetate, diethyleneglycol, various polyethylene-glycols, dipropyleneglycol, various polypropyleneglycols, dimethylsulfoxide, N-methylpyrrolidone, various mixtures of the above, and the like. The specifically preferred solvent for use in the practice of the invention is sulfolane.

The aromatic selectivity of the solvent can usually be further enhanced by the addition of water to the solvent@ without reducing substantially the solubility of the solvent phase for aromatic hydrocarbon. The presence of water in the solvent composition further provides a relatively volatile material therein which is distilled from the solvent in the extractor stripper, more fully discussed hereinafter to vaporize the last traces of nonaromatic hydrocarbons from the solvent stream by steam distillation. The composition of the present invention preferably contains from about 0.5% to about 20% by weight water and preferably from about by weight depending upon the particular solvent utilized and the process conditions at which the extraction zones and extractive stripper are operated.

The extraction zone of the present invention is operated at elevated temperature and sufliciently elevated pressure to maintain the feedstock, the solvent and reflux streams in liquid phase. Typically, suitable temperatures when using sulfolane as the solvent are within a range from about 80 F. to about 400 F. and, preferably, from about 175 F. to about 300 F. Similarly, suitable pressures are generally within the range from about atmospheric pressure up to about 400 p.s.i.g. and, preferably, from 50 to 150 p.s.i.g.

It is noted from the description of the present invention thus far that one of the critical problems to which this invention is directed is the displacement of non-aromatic hydrocarbons from the extract phase at the lower end of the extraction zone by utilizing the technique of a specific nonaromatic hydrocarbon containing reflux at that point. It is distinctly preferred that this reflux stream cornprise relatively light non-aromatic hydrocarbons and may contain significant quantities of aromatic hydrocarbons, e.g. from 30% to 60% by weight. In other words, the preferred practice of this invention is to operate a second distillation column in such a manner that a reflux stream is obtained which comprises a concentrate of the lightest nonaromatic hydrocarbon components which were left in the extract phase from the extraction zone. In conjunction therewith, the present invention provides operating flexibility wherein relatively heavy nonaromatic hydrocarbons may also be obtained either as a separate product stream and/or utilized as an additional nonaromatic hydrocarbon reflux in the extraction zone, details of which will be more fully discussed hereinbelow. The ability to tailor the physical characteristics of the reflux stream to the needs within the extraction zone for a displacing agent permits increased efticiencies to be obtained in the solvent extraction technique whereby increased yields and purity of aromatic hydrocarbons will result therefrom.

The amount of reflux introduced into the lower end of the extraction zone may be Varied considerably depending upon the degree to which nonaromatic hydrocarbons are being rejected in the extraction zone. It is preferred that the relatively light non-aromatic hydrocarbon reflux introduced into the lower end of the extraction zone be at least 10% by volume of the extract phase leaving the extraction zone in order to effectively displace nonaromatic hydrocarbons from the extract phase into the raffinate phase.

The first distillation column which is utilized in the processing train of the present invention is operated at moderate pressures and substantially high reboiler temperatures to force all of the non-aromatic hydrocarbon material and a portion of the aromatic hydrocarbons, water, and solvent out of the bottom portion of this column. It is preferred that the rst distillation column be operated substantially as a stripper column; that is, the extract phase should be fed into the rst distillation column at the upper portion thereof in order to provide maximum stripping to take place in the column. In order for extremely high purity aromatic hydrocarbons to be ultimately obtained it is essential that this first distillation column be operated in a manner to prevent all eXCePt minute traces of nonaromatic hydrocarbons from being withdrawn from the bottom of this column.

Typically, the extractive stripper will be maintained under pressures from atmospheric to about p.s.i.g.; although, generally, the top of the stripper column is maintained at from l to about 20 p.s.i.g. The reboiler temperature maintained in the extractive stripper is, of course, dependent upon the composition of the feedstock and the solvent. The column is arranged to take only an overhead fraction and a bottoms fraction as separate product streams. It is also within the concept of the present invention that a stacked vessel design be used, i.e. the second distillation zone may be stacked on top of the extractive stripper using a conventional total liquid trap-out tray between the two zones. Those skilled in the art, from a knowledge of the prior art, are well versed in the operation of a stripper column of this type and, therefore, additional details thereof need not be presented here.

The aromatic and solvent recovery column is operated at low pressures and suthciently high temperatures to distill the aromatic hydrocarbons overhead as a distillate fraction, thereby producing a bottoms 4fraction comprising lean solvent which is generally suitable for reuse in the solvent extraction zone. Again, the choice of operating conditions depends upon the feedstock characteristics and the composition of the solvent. Preferably, the top of the aromatic recovery column is operated at about 100 to 400 mm. absolute. These sub-atmospheric pressures must be employed in order to maintain a sufficiently low reboiler temperature to avoid thermal decomposition of the solvent, particularly when the solvent is of the sulfolane-type. Preferably, the reboiler temperature should vbe maintained below about 360 F. when using saturated sulfolane as the solvent.

The second distillation zone or column, which is an essential part of the present invention, may be operated under conditions of temperature and pressure which are suilicient to separate the overhead fraction from the extractive stripper column into a distillate fraction comprising non-aromatic hydrocarbons and a bottoms fraction which is essentially solvent, but may contain aromatic hydrocarbons again suitable for reuse in the extraction zone. In another preferred embodiment of the invention, the distillation conditions for this essential fractionation zone are suicient to produce at least one sidecut fraction comprising relatively heavy non-aromatic hydrocarbons which may then be recovered as a separate product stream or in whole or in part as an additional nonaromatic reflux in the extraction zone. The temperature and pressure conditions for this column desirably may be essentially the same as those present in the upper portion of the first distillation column; that is, in a distinctly pre- -ferred manner of operation, the overhead fraction from the extractive stripper is obtained as a vapor stream and is passed in its vapor state directly into the second distillation column for yfractionation therein into the various desired fractions previously discussed. Those skilled in the distillation art will know from the teachings presented herein and from general knowledge of the art exactly what conditions of temperature and pressure to maintain in this distillation column to achieve the desired results.

Suitable operating conditions for stripper 17 include a top temperature from 190 F. to 290 F., typically 250 F.; a top pressure from 6` p.s.i.a. to 361 p.s.i.a., typically 2l p.s.i.a.; a bottom temperature from 300 F. to 400 F., typically 310 F.; and, a bottoms pressure from 10 p.s.i.a. to 50 p.s.i.a., typically 25 p.s.i.a.

Similarly, operating conditions for fractionator 26 may include a column top temperature from F. to 230 F., typically 180 F.; a column top pressure from 2 p.s.i.a. to 42 p.s.i.a., typically 17 p.s.i.a.; a column bottoms temperature from 180 F. to 280 F., typically 230 F.; and, a column bottoms pressure from 4 p.s.i.a. to 44 p.s.i.a., typically 19 p.s.i.a. Those skilled in the art from general knowledge and the teachings presented herein will know how to choose the proper operating conditions from tower 17 and 26.

The invention may be more fully understood with reference to the appended -drawing which is a schematic representation of apparatus for practicing one embodiment of the invention.

DESCRIPTION OF THE DRAWING Referring now to the drawing, a hydrocarbon -feed mixture obtained as the conventional mixed product from a catalytic reforming unit and, typically, comprising benzene, toluene, and xylenes mixed with corresponding naphthenes and paraflins enters the system via line into extractor 11 at generally an intermediate location therein. Aqueous sulfolane as the lean solvent enters the upper portion of extractor 11 via line 12 and a relatively light reflux stream, more fully discussed hereinafter, enters extractor 11 at a lower end thereof via line 13. Desirably, a relatively heavy reflux stream, more fully discussed hereinafter, also enters extractor 11 via line 14, which may be located above or below feed line 10. An extract phase comprising solvent having dissolved therein aromatic hydrocarbons and also which is contaminated with nonaromatic vhydrocarbons is withdrawn from extractor 11 via line 16. A rainate stream comprising solvent and a major portion of the naphthenes and parains originally present in the feed is removed from extractor 11 via line 15 for further processing by means not shown.

The extract stream in line 16 is passed into extractive stripper column 17 at the upper end thereof. Additional solvent may be added to column 17 as needed from a source not shown in order to enhance the separation of the nonaromatic hydrocarbons from the solvent containing the aromatic hydrocarbons. Sufficient `operating conditions are maintained in extractive stripper 17 to produce a bottoms fraction comprising solvent having dissolved therein the desired aromatic hydrocarbons which are removed from stripper 17 via line 18.

A distillate fraction in vapor form and comprising nonaromatic hydrocarbons and minor amounts of solvent is removed via line and preferably passed directly in its vapor state into second distillation column 26.

Fractionator 26 is maintained under conditions sufficient to produce a distillate fraction comprising relatively light non-aromatic hydrocarbons which are removed via line 13, a side-cut fraction comprising relatively heavy non-aromatic hydrocarbons which are removed via line 14, and a bottoms fraction comprising lean solvent substantially free of aromatic and nonaromatic hydrocarbons and which is withdrawn via line 27.

The relatively light nonaromatic hydrocarbons present in line 13 are passed into extractor 13 at the lower end thereof as reflux in the extraction zone in an amount sufficient t-o act as the displacing agent for non-aromatic hydrocarbons present in the solvent at that lower portion of the extraction zone. Desirably, at least a portion of the relatively heavy non-aromatic hydrocarbons are passed via line 14 as heavy reflux into extractor 11. This heavy reux may be introduced into extractor 11 at any point in the extraction zone between the entry point of light reflux in line 13 and the entry point of lean solvent from line 12. If heavy reux is needed or desired on extractor 11, it should, typically, be introduced into extractor 11 at a locus intermediate between the feed point and the light reflux point.

In many cases, it is preferable to withdraw from the system a relatively heavy non-aromatic hydrocarbon product stream via line 28. Similarly, the lean solvent present in line 27 may be withdrawn from the system for storage purposes, regeneration purposes, or in whole or in part may be returned to the extractor 11 via lines 29, 23, and 12. An acceptable place to return this stream to the extractor may be through line 14 and/or line 10.

Returning now to extractive stripper 17, the bottoms fraction in line 18, as previously mentioned, contains solvent having dissolved therein substantially all of the aromatic hydrocarbons which were originally present in the feed. This bottoms fraction is now passed into fractionator 19 which is maintained under conventional distillation conditions as set forth hereinabove. The aromatic hydrocarbons are removed from fractionator 19 via line 20 in high concentration and in extremely high purity. The bottoms material from fractionator 19 comprises lean solvent generally suitable for reuse in the extraction zone and is withdrawn from fractionator 19 via line 21. Since the selective solvent, particularly in the case of sulfolane, does become contaminated during processing, a drag stream thereof isl frequently taken via line 22 to solvent regeneration means, not shown, in order to remove contaminating impurities from the system. The remainder of the material from the bottom of fractionator 19 is returned to extractor 11 via lines 23 and 12. Makeup lean solvent, as needed, may be introduced into the system via line 24.

PREFERRED EMBODIMENT Therefore, from the presentation presented hereinabove, the preferred embodiment of the invention provides a process for separating and recovering single ring aromatic hydrocarbons from a feed mixture containing aromatic and non-aromatic hydrocarbons which comprises the steps of: (a) introducing said feed mixture into a vertically disposed extraction column at an intermediate location, passing lean solvent selective for aromatic hydrocarbons into the upper end of said column, introducing hereinafter specified relatively light reflux into the lower end of said column, said extraction conditions being suficient to produce a raffinate stream comprising solvent admixed with non-aromatic hydrocarbons and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and contaminated with non-aromatic hydrocarbons; (b) withdrawing said raffinate stream from the top of said column and withdrawing said contaminated extract stream from the bottom of said column; (c) passing s-aid extract stream into the upper portion of a first distillation column maintained under conditions sufiicient to produce a first overhead fraction comprising solvent and nonaromatic hydrocarbons and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein; (d) passing said first bottoms fraction to a separation zone maintained under conditions sufficient to produce a product stream comprising single ring aromatic hydrocarbons and a solvent stream suitable for reuse in said extraction column; (e) introducing said first overhead fraction into a second distillation zone maintained under distillation conditions sufficient to produce a second overhead fraction comprising relatively light nonaromatic hydrocarbons and a second bottoms fraction comprising -solvent suitable for reuse in said extraction column; (f) passing at least a portion of said second overhead fraction into said extraction column as the specified reflux stream of Step (a) and, (g) recovering said product stream of step (d).

An additionally preferred embodiment of the present invention is the preferred process hereinabove wherein said second distillation conditions are sufficient to produce additionally a side-cut fraction comprising relatively heavy nonaromatic hydrocarbons.

A distinctly preferred embodiment of this invention is the proce-ss described hereinabove wherein the selective solvent for aromatic hydrocarbons comprises sulfolane.

The invention claimed:

1. Process for separating aromatic hydrocarbons from a feed mixture containing aromatic and nonaromatic hydrocarbons which comprises:

(a) introducing said feed mixture into a solvent extraction zone in contact with lean solvent and hereinafter specified reflux stream under conditions sufficient to produce a rafiinate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing nonaromatic hydrocarbons;

(b) removing from said zone the raffinate stream;

(c) introducing said extract stream into a first distillation zone maintained under distillation conditions;

(d) withdrawing from said first distillation zone a first overhead fraction containing solvent and nonaromatic hydrocarbons and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(e) passing said rst overhead fraction into a second distillation zone maintained under conditions suflicient to produce a distillate fraction containing nonaromatic hydrocarbons and a second bottoms fraction comprising solvent; and

(f) returning said distillate fraction to said extraction zone as the 4specified reux therein.

2. Process according to claim 1 wherein said lean solvent comprises sulfolane.

3. Process according to claim 2 wherein said aromatic hydrocarbons comprise single ring type aromatic hydrocarbons.

4. Process for separating and recovering single ring aromatic hydrocarbons from a feed mixture containing aromatic and nonaromatic hydrocarbons which comprises the steps of:

(a) introducing said feed mixture into a vertically disposed extraction column at an intermediate location, passing lean solvent selective for aromatic hydrocarbons into the upper end of said column, introducing hereinafter specified relatively light reflux into the lower end of said column, said extraction conditions being suiiicient to produce a rainate stream comprising solvent admixed with non-aromatic hydrocarbons and an extract stream comprising solvent having aromatic lhydrocarbons and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and contaminated with nonaromatic hydrocarbons;

(b) withdrawing said raffinate stream from the top of said column and withdrawing said contaminated extract stream from the bottom of said column;

(c) passing said extract stream into the upper portion of a first distillation column maintained under conditions sufficient to produce a first overhead fraction comprising solvent and non-aromatic hydrocarbons and a first bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(d) passing said first bottoms fraction to a separation zone maintained under conditions sufficient to produce a product stream comprising single ring aromatic hydrocarbons and a solvent stream suitable for reuse in said extraction column;

(e) introducing said first overhead fraction into a second distillation Zone maintained under distillation conditions suiiicient to produce a second overhead fraction comprising relatively light nonaromatic hydrocarbons and a second bottoms fraction comprising solvent suitable for reuse in said extraction column;

(f) passing at least a portion of said second overhead fraction into said extraction column as the specified reux stream of step (a); and

(g) recovering said product stream of step (d).

5. Process according to claim 4 wherein said second distillation conditions are sufficient to produce, additionally, a side-cut fraction comprising relatively heavy nonaromatic hydrocarbons.

6. Process according to claim S wherein said side-cut fraction is returned to said extraction column at an intermediate location therein.

7. Process according to claim 5 wherein said lean solvent comprises sulfolane.

References Cited UNITED STATES PATENTS 2,730,558 1/1956 Gerhold 208--321 2,838,582 6/1958 Kassel et al. 208--318 2,886,610 5/1959 Georgian 208-321 3,179,708 4/1965 Penisten 208-321 3,262,875 7/1966 Girotti et al. 208-321 3,396,101 8/1968 Broughton 208-318 HERBERT LEVINE, Primary Examiner U.S. Cl. X.R. 

